1. Field of the Invention
The present invention relates generally to an amplification system for mobile communication, applied to a communication system, and in particular, to a Class-S amplifier system for efficiently amplifying a signal having a high Peak to Average Power Ratio (PAPR) in a communication system.
2. Description of the Related Art
Generally, power amplifiers used in a communication system are identified according to their classes, and the classes are defined in light of operation duration and a bias current of an output device. In that light, power amplifiers are classified as Class A, Class B, Class C, Class AB, Class F, and Class S. Each of the classes of the power amplifiers will be described in brief hereinbelow. A power amplifier whose operating point is put on the center of the bias is classified as Class A, and a power amplifier whose operating point is put on a 0V (zero volt)-bias is classified as Class B. A power amplifier whose operating point is put between Class A and Class B is classified as Class AB, and a power amplifier in which harmonic matching is added to an output matching stage is classified as Class F. Finally, a power amplifier that amplifies an input pulse signal in the form of a pulse is classified as Class S.
Herein, a description will be made of a Class-S system among the amplification systems of the various classes, and a Class-S amplifier used in the Class-S system.
The Class-S amplifier system converts a Radio Frequency (RF) signal into an RF pulse signal using a delta-sigma modulator. The delta-sigma modulated signal is amplified into an RF pulse signal using a power amplifier operating in a switching mode. Subsequently, the amplified signal is restored to its original signal after a switching harmonic component is removed therefrom through a band-pass filter. In this case, the power amplifier, as it operates in the switching mode, theoretically has an efficiency of 100%. In addition, a nonlinear component generated by the delta-sigma modulator and the RF amplifier is removed through a separate linearizer.
With reference to FIG. 1, a detailed description will now be made of a structure of the Class-S amplification system.
FIG. 1 is a block diagram schematically illustrating a structure of a Class-S system using a general band-pass delta-sigma modulator.
Referring to FIG. 1, the general Class-S system includes a delta-sigma modulator 101, and a Class-S amplifier 107 composed of a power amplifier 103, and a band-pass filter 105.
The delta-sigma modulator 101 converts an input RF signal into an RF pulse signal using delta-sigma modulation, and outputs the RF pulse signal to the power amplifier 103. The power amplifier 103 operating in the switching mode amplifies the RF pulse signal received from the delta-sigma modulator 101 to a required level set in the system, and outputs the amplified RF pulse signal to the band-pass filter 105. The band-pass filter 105 removes a switching harmonic component included in the received amplified RF pulse signal to restore the RF pulse signal to its original signal. In this case, the power amplifier 103, as it operates in the switching mode, theoretically has an efficiency of 100%. Although not illustrated in the drawing, the Class-S system can include a separate linearizer for removing a nonlinear component generated by the delta-sigma modulator 101 and the power amplifier 103.
In order to generate the RF pulse signal, the conventional amplification system uses an over-sampling Analog-to-Digital Converter (ADC) such as the band-pass delta-sigma modulator. However, in the common mobile communication system, an RF signal has a frequency of 800 MHz or higher. Therefore, the conventional amplification system undesirably needs a band-pass delta-sigma modulator of 4-times over-sampling, i.e. over-sampling of about 3.2 GHz. For example, for a band of the IMT-2000 communication system, there is need for a fast band-pass delta-sigma modulator of about 8 GHz or higher.
In addition, the switching-mode power amplifier 103 should undesirably operate at up to 5 times the input RF frequency, i.e. have a broadband characteristic, for accurate amplification of the RF pulse signal output from the delta-sigma modulator 101. In other words, for a band of the IMT-2000 system, there is a need for a switching-mode power amplifier operating at about 10 GHz. However, it is difficult to actually implement the fast band-pass delta-sigma modulator and the switching-mode power amplifier, and they are expensive. In addition, it is also hard to actually implement a method of matching an input impedance of the power amplifier for the broadband input signal.
Aside from the block diagram of the power amplification system using the delta-sigma modulator, FIG. 1 shows signal flow diagrams generated at outputs of respective blocks, i.e. shows a signal flow in frequency and time domains.
As illustrated in FIG. 1, the delta-sigma modulator 101 converts a random input signal into a Pulse Width Modulation (PWM) signal having a constant envelope, generating quantization noises at an outer band of the signal. The power amplifier 103 amplifies the intact delta-sigma modulated signal at high efficiency. The power amplifier 103 performs linear amplification on the output signal of the delta-sigma modulator 101, having the constant envelope. To achieve high efficiency, the power amplifier 103 may be a Class-F amplifier.
Ideally, the Class-F amplifier has an efficiency of 100%. Therefore, the band-pass filter 105 receives the output signal of the power amplifier 103, and suppresses noises at the outer band of the signal to extract only the amplified original signal. One of the most important considerations in the amplification system is to deliver the delta-sigma modulated broadband signal to the power amplifier without distortion.
In order to increase a Signal-to-Noise Ratio (SNR), the conventional Class-S system using the delta-sigma modulator converts, in the time domain, a pulse wave into a signal having a bandwidth much broader than that of the original signal in the frequency band using an over-sampling technique and a noise shaping technique. In this case, if a baseband delta-sigma modulator is used, the broadband signal should be delivered from a digital Intermediate Frequency (IF) stage to the power amplifier via an RF transceiver without distortion. In addition, if the band-pass delta-sigma modulator is used, the broadband signal should be delivered from an output of the delta-sigma modulator to the power amplifier without distortion.
Although there is a theoretical description of a method for delivering transmission signals to the power amplifier without distortion, the conventional Class-S system does not present a detailed implementation method thereof and the possible problems occurring when actually implemented. That is, although implementation of the delta-sigma modulator and the power amplifier is possible, there is no proposed solution for combining them and reducing distortion of the signals delivered to the power amplifier.
That is, the conventional amplification system using the delta-sigma modulator has no scheme capable of delivering the delta-sigma modulated constant envelope signal to the power amplifier without distortion. Therefore, the power amplifier receives a signal not having a constant envelope. As a result, the power amplifier decreases in efficiency, for linear power amplification, causing deterioration of the entire performance of the delta-sigma modulation system.